Changes in chromosomal dosage produce severe aneuploid syndromes and are a major contributor to birth defects. It is believed that the basis for these syndromes is the fact that gene regulatory systems in higher eukaryotes are dosage dependent. The major type of dosage effect is a negative one in that there is an inverse correlation of target gene expression with the dosage of a varied regulator. An exception to aneuploid syndromes associated with altered chromosome numbers is dosage compensation. The inverse dosage effect has been implicated as the basis of X chromosome dosage compensation in Drosophila. However, the dosage effect on the autosomes has been muted during the evolution of the sex chromosomes by the sequestration of a histone acetylase to the X via the action of the male specific lethal (MSL) complex. This complex is localized to the X chromosome in males and spreads from nucleation sites to cover the X. In the proposed work, transformed constructs of nucleation sequences will be used to determine the determinants and constraints of spreading of the MSL complex. Secondly, the accumulated data suggest that the MSL complex is capable of overriding an impact on gene expression of either increases or decreases of histone modification. Histone modifications are generally thought to open chromatin for access by transcription factors. This unique property of the MSL complex will be [unreadable] further investigated using a newly developed fluorescent RNA in situ gene expression assay for embryos. Genotypes with altered histone acetylase activity and distribution with or without the MSL complex will be examined for the impact on X and autosomal gene expression. The MOF histone acetylase or the MSL complex with or without the other component will be targeted to a mini-white construct to test the interaction of histone modification versus the MSL complex on gene expression. Lastly, target gene expression will be assayed in mutant combinations of other chromatin components implicated in interacting with the MSL complex. The proposed project will contribute to an understanding of how chromatin modifications become established along the length of the chromosome and how they affect gene expression. [unreadable] [unreadable]